Electrographic developing method suited for transfer electrophotography without cleaning

ABSTRACT

A METHOD OF TONING AN ELECTROSTATIC IMAGE ON A SURFACE IN TWO STEPS: (1) BRUSHING, RUBBING OR OTHERWISE CONTACTING THE IMAGE-BEARING SURFACE WITH FIBERS, PARTICLES OR OTHER MATERIALS CAPABLE OF PROPERLY TRIBOELECTRICALLY CHARGING TONER ON THE SURFACE TO THE PROPER POLARITY IN PRESENCE OF A FIELD ADAPTED TO REPEL IMPROPERLY CHARGED TONER FROM THE SURFACE, AND (2) TONING THE IMAGE, FOR EXAMPLE, BY BRUSHING. THIS METHOD IS PARTICULARLY SUITED TO REPETITIVE TRANSFER XEROGRAPHY WITHOUT CLEANING OF THE SURFACE OF THE PHOTOCONDUCTIVE ELEMENT.

June 29, 1971 I. B. VILLE 3,589,895 ELECTROGRAPHIC DEVELOPING METHOD SUITED FOR TRANSFER ELECTROPHOTOGRAPHY WITHOUT CLEANING Filed July 17, 1967 2 Sheets-Sheet l FIG.2A

III

FIG.2C

IVAN B. VILLE INVENTOR.

ATTORNEYS June 29, 1-971 3,589,895

' ELECTROGRAPHIC DEVE ING METHOD SUITED FOR TRANSFER BLECTROPHOTOGRAPHY WITHOUT CLEANING Filed July 17, 1967 2 Sheets-Sheet 2 IVAN B. VILLE INVENTOR.

M75 1 KW 641 44;,

ATTORNEYS United States Patent 01 lice 3,589,895 Patented June 29, 1971 ELECTROGRAPHIC DEVELOPING METHOD SUITED FOR TRANSFER ELECTROPHOTOG- RAPHY WITHOUT CLEANING Ivan B. Ville, Rochester, N.Y., assiguor to Eastman Kodak Company, Rochester, N.Y. Filed July 17, 1967, Ser. No. 653,934 Int. Cl. G03g 13/22 U.S. CI. 96-14 14 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATED APPLICATION Reference is made to cofiled application, Ser. No. 653,779, filed July 17, 1967 in the name of Jarvis, J. G. and York, W. C. entitled Method of Repetitive Xerography Without Cleaning.

BACKGROUND OF THE INVENTION This invention relates to electrography and, more particularly, to a method and apparatus for development of electrostatic charge patterns on an unclean surface.

In certain electrographic processes an electrostatic charge pattern is formed on a charge-holding surface, it is developed by toning and the resulting toner pattern is transferred to a receiving surface. For example, in basic transfer xerography, the formation of the charge pattern is accomplished photographically by uniformly charging a photoconductive surface of a xerographic plate and imagewise exposing that plate, thereby discharging the areas exposed, i.e., the background areas when a positive original is used, and leaving a charge in the areas not exposed, i.e., the image areas.

After transfer of the toner, the charge-holding surface is reused. Because common transfer methods do not transfer all of the toner, it has been the practice to clean the charge-holding surface before reuse. Previously mentioned cofiled Jarvis et al. application teaches that cleaning can be eliminated by exposure through the base and redistribution of residual toner at the toning station. The rnethods disclosed therein are quite effective for processes using reverse development. With many toners in a positive-to-positive process, there is a tendency of some of the toner to remain in the background, giving a final print that, although readable, is not of the highest quality.

SUMMARY OF THE INVENTION It is an object of this invention to improve on processes which eliminate the necessity of cleaning the chargeholding surface in electrophotography thereby reducing waste of toner and abrasion of the charge-holding surface.

.It is another object of this invention to effectively tone an electrostatic charge pattern carried on a surface which already carries toner in both charged and relatively discharged areas.

It is another object of this invention to provide xerographic prints having sharp contrast and clean background.

It is another object of this invention to eliminate the necessity for an exceptionally high-quality cleaning operation in electrostatic printing or copying apparatus.

Formation of a charge pattern on a surface despite residual toner on such surface can be obtained by several processes. For example, patterned corona charging from a shaped stylus can be effective despite some residual toner. Similarly, as in said Jarvis application, exposure can be accomplished through the base or support of a transparent photoconductive member rather than from the charge-holding side where residual toner could interfere.

In attempting to tone charge patterns so formed in direct positive-to-positive processes, I found that ordinary toning methods having a tendency to develop poor contrast and unclean backgrounds after a number of prints have been made. The poor results are due to a reluctance of the residual toner to leave the background areas when subjected to an ordinary toning operation and are not substantially eliminated by merely increasing the magnitude of toning, for example, by using two or more ordinary magnetic brushes.

1 have solved this problem with a two-step toning operation in which an electrical field, in the first step, aids proper charge of the toner and, in the second step, aids cleaning of the background.

More specifically, in the first step, residual toner particles on the charge pattern are brushed, rubbed or otherwise contacted by a material capable of properly charging said particles by triboelectric effect in the course of said contacting. This contacting is done in the presence of an electric field of a direction such that toner particles so charged are urged toward both background and image areas of the charged surface. More important, such a field will urge particles having an improper charge away from said charge pattern and toward the contacting material where they can be properly charged. For example, the first step can be carried out by brushing the image with a magnetic brush similar to those presently used in the art except that it is biased with a potential of polarity opposite to that of the charge pattern relative to the discharged areas. Since many xerographic exposing steps do not completely eliminate charge in the exposed areas, grounding the brush will often create the proper field.

The second step is an ordinary toning step of the type adapted to both tone the charge areas and clean the uncharged or less charged areas. For example, the pattern can be brushed with a magnetic brush to which toner has been applied. In order to clean the less-charged areas of properly charged toner, a small bias is applied to such a brush of the same polarity as the charge pattern. This, in combination with the charge pattern, creates an electrical field having a direction that will urge properly charged toner away from the background or less-charged areas but toward image or greater-charged areas. These two steps give high quality prints with a clean background in a process in which ordinary toning gives unclean background prints.

Although not essential to obtaining the remarkable results mentioned above, still better results are obtained if the first step is carried out in a condition of low toner concentration, for example, with a toner-starved magnetic brush.

It is another feature of the invention that the toner concentration on the material used in carrying out the first step may be so chosen relative to the toner already on the photoconductive surface that an equilibrium between toner deposited and toner removed is maintained, thereby eliminating the necessity for replenishing or removing toner in the first toning step.

In a xerographic embodiment of the invention, the electrostatic image is created by exposing through the base, a uniformly charged xerographic plate, the photoconductive surface of which has not been cleaned. The resulting electrostatic image is first brushed with a grounded magnetic brush with a small enough concentration of toner that its bristles attract new toner. This brushing imparts the proper charge to the toner by triboelectric action and at least partially redistributes the toner to the image areas. The image is then brushed with a second magnetic brush having a higher concentration of toner and an electrical bias of the same polarity as the image, thereby giving a high-contrast toner image with clean background. This image is transferred and the process repeated without cleaning the photoconductive surface. Substantial waste of toner and scratching of the photoconductor are sharply reduced over present transfer xerographic processes.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A, 1B and 1C are diagrammatic illustrations showing use of the inventive process with an already formed electrostatic image.

FIGS. 2A, 2B and 2C are diagrammatic illustrations showing the use of the inventive process in xerography.

FIG. 3 is a schematic diagram of an automatic electrophotographic printer using the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1A, 1B and 10, an insulating material 1 having a conductive backing 9 carries a charge pattern 2 on a surface 7. The surface 7 having not been cleaned also carries toner particles 3 corresponding to the previous copy, generally adhering both in areas of charge and in areas without charge. The surface 7 is toned in two steps. In the first step, the surface is rubbed, brushed or otherwise contacted with a material adapted to triboelectrically charge the toner particles 3 to a polarity making them attractable to the charge pattern 2. In FIG. 13, this step is shown being carried out with bristles 6 of a brush 4. This step is carried out in the presence of an electric field having a polarity such that it urges a toner particle properly charged toward both areas of high charge and areas of relatively low charge, and urges improperly charged toner away from the charge pattern. In FIG. 1B, the field is shown being provided by a bias of polarity opposite to the charge pattern applied to a conductive part of the brush 4 from a voltage source 8.

The second step of the toning operation is shown in FIG. 10, where the surface is toned in much the usual manner. For example, in FIG. 10 the surface 7 is toned by brushing with a brush carrying a normal amount of toner and similar to the brush 4 used in the first step but in the presence of an electric field which urges the toner toward the charged areas but away from the uncharged areas. This electrical field is created by a slight bias applied to a conductive part of the brush 5 from voltage source 10 which bias has the same polarity as the charge pattern 2 but is of less magnitude.

In FIGS. 2A, 2B and 2C this process is applied to ordinary transfer xerography. In FIG. 2A, a transparent support member 20 has coated thereon, in order, a transparent conductive layer 21 and a photoconductive layer 22. The free surface 24 of the photoconductive layer has been used and has not been cleaned, so it carries residual toner 3. This surface 24 is given a uniform charge by a corona charger 23.

As shown in FIG. 2B, the photoconductive layer 22 is imagewise exposed through the support layer 20 and conductive layer 21, thereby dissipating most of the charge in the areas exposed and leaving a charge pattern on the surface 24 of the photoconductive layer.

As shown in FIG. 2C, the surface 24 of the photoconductive layer is toned by brushing twice, the first brushing being with a magnetic brush 25 containing bristles triboelectrically attractive to the toner and having a tendency in the brushing action to charge the toner to a polarity opposite that of the charge on the photoconductive surface 24. This first brushing is done in the presence of a field which has a tendency to urge toner so charged toward both background and image areas of the photoconductive surface 24. More important, it has a tendency to urge toner not so charged away from both background and image areas, where it can be charged properly. Since, in many processes, some residual charge remains in the background areas, this field often can be created by grounding the magnetic brush 25. The second step of the toning operation is accomplished by brushing with a second magnetic brush'26 which applies toner in the conventional manner in the presence of an electric field, such as that created by voltage source 27 connected to the brush 26, which has a tendency to urge toner away from the background areas.

An example of the type of brush that is particularly suited for both steps of this invention is the magnetic brush shown in US. Pat. No. 2,874,063, Greig. Such a brush uses a developer containing fine resin toner particles combined with much larger ferromagnetic carrier particles. The carrier particles are held in bristle formation by a magnetic field commonly imposed from within the brush. The toner is given the proper charge by rubbing with the carrier particles.

The following technical explanation for the remarkable results mentioned above is not intended to limit the scope of the invention but rather to suggest a reason such results are believed obtained. In positive-to-positive systems, ordinary field development, for example with a magnetic brush, works best with a development electrode having a small bias which creates an electrical field urging toner particles away from background areas of a charge pattern to be developed. For example, if the surface potential in the image areas of the charge pattern is minus 500 volts and that of the background areas is minus 50 volts, for direct development a bias of a magnitude between these two potentials, is generally placed on the magnetic brush, say minus volts. Positive toner particles are then urged away from the background areas but toward the image areas, giving a clean background and high contrast. If, however, residual toner is present and has picked up an improper charge (negative, in this case), for example, by having undergone a negative charging step, the field will urge such toner toward the background areas. This is borne out by the fact that prints having an unclean background are commonly obtained using the steps shown in FIG. 2 but with both brushes biased according to the prior art. Modifying the electrical field, for example by grounding the brush 25 in FIG. 2C, to a point where properly charged toner is urged to both background and image areas has a tendency to loosen any toner retaining a charge of an unwanted polarity. This improperly charged toner is attracted to the brush Where it can be properly charged and redeposited on the image pattern. Although this step leaves a number of properly charged toner particles in the background areas, they are cleaned off or distributed to image areas with the second toning step.

When a brush is used in the first toning step, the toner concentration on the brush can be maintained in an equilibrium condition so that there is little net loss or gain of toner with continued use. Although no toner needs to be on the first brush for it to operate properly in the process, it is found that the process works best with a small amount of toner so that the brushing action in the first step is essentially a redistributing action rather than either a toning or a cleaning action alone. If the process is begun with no toner on the first brush, it will gradually pick up toner from the photoconductive surface until it approaches an equilibrium concentration and will tend to maintain that concentration without further adjustment or addition of toner.

As an example of the invention, a xerographic plate having a photoconductor receptive to a negative charge is uniformly charged to a potential of minus 500 volts with respect to its conductive layer which is grounded. The plate is then imagewise exposed through the support resulting in an electrostatic image having a surface charge with a potential of minus 400 volts in image areas ranging down to minus 50 volts in background areas. This image is brushed With a first magnetic brush having bristles three-eighths of an inch long, which bristles are electrically connected to ground. It is then brushed with a second magnetic brush having comparable bristles but with percent by weight of a fine toner mixed with the bristles. The second brush is connected to a negative potential of minus 150 volts. The toner image resulting from this second brushing is then transferred to a receiver (not shown) leaving some of the toner on the plate. Since this toner from the previous image is redistributed onto the next image area rather than being cleaned off, it can be seen that there is a significant saving in the amount of toner used. The process is repeated over and over again without cleaning. As more prints are made, the toner concentration of the first brush gradually builds up to about 2 percent, at which point it remains, without the necessity of adding or removing toner to or from the brush. As the toner builds up on the first brush, the background remains clean while the image gets slightly darker. Prints throughout are of excellent quality.

The potential on the second brush can be varied substantially from minus 150 volts with acceptable results. 'In fact, this brush may be allowed to float electrically Without an outside voltage being applied. More specifically, if the brush is insulated from ground, it electrically picks up an induced charge or voltage from the charge on the photoconductive layer, which will have a tendency to make it function toward toner in the same manner as though a voltage intermediate the image and background voltages Were actually directly applied to the brush.

This invention has particular application to an automatic electrophotographic printer. Referring to FIG. 3, a transparent drum 30, such as glass, has coated thereon a thin transparent conductive layer 31 and a photoconductive layer 32 which also can be transparent. In operation, the photoconductive layer 32 is given an electrostatic charge by a corona charger 33. It is then imagewise exposed by a suitable projector 34 from the inside of the drum leaving an imagewise charge on the surface of the photoconductive layer. This imagewise charge is toned by first brushing with a grounded magnetic brush 35 which can be starved of toner. It is then brushed with a second magnetic brush 36 carrying toner which brush has applied to it a small potential of the same polarity as that of the charge originally placed on the photoconductive layer by the corona charger 33. The resulting toner image is transferred to a receiving sheet 37 by a corona transfer device 38, and fused to the receiving sheet by a radiant heater 39. The drum is then reused without cleaning. A lamp 41 can be used to eliminate residual charge on the photoconductive layer prior to reuse.

Although no cleaning of the photoconductive surface is necessary, it may be desirable ot clean the inside of the drum of dust that may interfere with the next exposing step with a brush 40. Even with this cleaning station, use of the overall arrangement has a number of advantages over use of prior printers, including the elimination of scratching of the photoconductive layer as it is cleaned and a reduction in waste of toner.

In conjunction with the teachings of the Jarvis et al. invention mentioned above, this invention can be used to give either direct or reverse prints with the same equipment. For example, with an ordinary positive original and a positive polarity developer, a negative charge is placed on the photoconductive layer and the development biases are applied according to my invention. If a negative original is used, a positive charge is placed on the photoconductive layer, and the development biases are now adjusted for reverse development as taught in said Jarvis et al. application. These adjustments can all be made with a single switch at a control panel.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A method for developing an electrostatic charge pattern on a surface having residual toner particles of positive and negative polarity thereon, said charge pattern being of the type having image areas of a negative potential interspersed with background areas, said method comprising the steps of:

(a) first, brushing said surface in the presence of an electrical field directed toward said surface to urge positively charged residual toner particles towards said surface and to urge negatively charged residual toner particles away from said surface; and

(b) second, brushing toner bearing a charge of positive polarity onto image areas on said surface in the pres ence of electrical fields directed toward said image areas and away from said background areas.

2. A process according to claim 1 wherein said first brushing step is carried out with a brush having bristles triboelectrically related with respect to said toner so as to charge said toner to positive polarity and carrying toner in such an amount that there is no appreciable net deposit, of toner on said surface with repeated use of said surface.

3. A process according to claim 1 wherein said first brushing step is carried out by a grounded magnetic brush and said second brushing step is carried out by a magnetic brush biased to a negative potential magnitude between the potential magnitudes of said image and background areas.

4. A method for developing an electrostatic charge pattern formed by negatively charged areas of differing po tential on a surface containing random residual toner particles of positive and negative polarity, said method comprising:

(a) triboelectrically charging said residual toner particles to a positive polarity in a first electrical field directed toward said surface, and

(b) thereafter applying positively charged toner particles to and redistributing residual positively charged toner particles on said surface in an electrical field directed toward areas of said charge pattern having high negative potential and away from areas having low negative potential.

5. The method of toning an electrostatic charge pattern,

having image areas of greater and background areas of lesser negative charge, on a surface carrying residual toner from previous use, said method comprising the steps of: first, brushing said surface with a magnetic brush in the presence of a first electrical field directed toward said surface, said first field being of the type created by providing an electrical potential between said magnetic brush and said surface, and

thereafter applying with a magnetic brush toner bearing a charge of a positive polarity in the presence of a second electrical field, said second field being of the type created by providing an electrical potential between said magnetic brush and said surface, said brush being the negative terminal and said potential being of magnitude intermediate the charges of said image areas and said background areas so that said positive toner is urged toward said pattern areas of greater charge and away from said pattern areas of lesser charge.

6. A process of selectively applying toner charged to a positive polarity to an electrostatic charge pattern on a surface having thereon residual toner bearing a charge of positive polarity and residual toner bearing a charge of a negative polarity, said charge pattern having areas of high negative potential mixed with areas of low negative potential said process comprising the steps of:

(a) first, brushing said surface with a material having triboelectric characteristics attractive to said toner, said brushing step being carried out in the presence of an electrical field directed toward said surface; and

(b) thereafter applying toner bearing a charge of positive polarity to said surface in the presence of electrical fields directed toward said areas of high negative potential and away from said areas of low negative potential.

7. In a xerographic process which includes the steps of:

(a) forming an electrostatic image on a surface of a photoconductive layer by depositing on said surface a charge of negative polarity and imagewise exposing said photoconductive layer from the side opposite to said charge;

(b) applying positively charged toner to said electrostatic image to form a toner image;

(c) utilizing said toner image, leaving residual toner on said surface; and

(d) repeating steps (a), (b), and (c) using the same surface;

the improvement, eliminating a necessity of cleaning said surface between cycles, comprising, after said step of forming an electrostatic image, the added step of redistributing said residual toner from non-image areas to image areas of said surface by brushing said surface with bristles having a triboelectric attraction for said toner in the presence of an electrical field directed toward said surface.

8. A method for developing an electrostatic charge pattern on a surface having residual toner particles of positive and negative polarity thereon, said charge pattern being of the type having image areas of a positive potential interspersed with background areas, said method comprising the steps of:

(a) first, brushing said surface in the presence of a first electrical field directed away from said surface to urge negatively charged residual toner particles towards said surface and to urge positively charged residual toner particles away from said surface; and

(b) second, brushing toner bearing a charge of negative polarity onto image areas on said surface in the presence of electrical fields directed away from said image areas and toward said background areas.

9. A process according to claim 8 wherein said first brushing step is carried out with a brush having bristles triboelectrically related with respect to said toner so as to charge said toner to negative polarity and carrying toner in such an amount that there is no appreciable net deposit of toner on said surface with repeated use of said surface.

10. A process according to claim 8 wherein said first brushing step is carried out by a grounded magnetic. brush and said second brushing step is carried out by a magnetic brush biased to a positive potential magnitude between the potential magnitudes of said image and background areas.

11. A method for developing an electrostatic charge pattern formed by positively charged areas of differing potential on a surface containing random residual toner particles of positive and negative polarity, said method comprising:

(a) triboelectrically charging said residual toner particles to a negative polarity in a first electrical field directed away from said surface, and

(b) thereafter applying negatively charged toner particles to and redistributing residual negatively charged toner particles on said surface in an electrical field directed away from areas of said charge pattern having high positive potential and towards areas having low positive potential.

12. The method of toning an electrostatic charge pattern, having image areas of greater and background areas of lesser positive charge, on a surface carrying residual toner from previous use, said method comprising the steps of:

first, brushing said surface with a magnetic brush in the presence of a first electrical field directed away from said surface, said first field being of the type created by providing an electrical potential between said magnetic brush and said surface, and

thereafter applying with a magnetic brush toner bearing a charge of a negative polarity in the presence of a second electrical field, said second field being of the type created by providing an electrical potential between said magnetic brush and said surface, said brush being the positive terminal and said electrical potential being of magnitude intermediate the charges of said image and background areas so that said negative toner is urged toward said pattern areas of greater charge and away from said pattern areas of lesser charge.

13. A process of selectively applying toner charged to a negative polarity to an electrostatic charge pattern on a surface having thereon residual toner bearing a charge of positive polarity and residual toner bearing a charge of a negative polarity, said charge pattern having areas of high positive potential mixed with areas of low positive potential said process comprising the steps of:

(a) first, brushing said surface with a material having triboelectric characteristics attractive to said toner, said brushing step being carried out in the presence of 5m electrical field directed away from said surface; an

(b) thereafter applying toner bearing a charge of negative polarity to said surface in the presence of electrical fields directed away from said areas of high positive potential and toward said areas of low positive potential.

f14. In a Xerographic process which includes the steps 0 (a) forming an electrostatic image on a surface of a photoconductive layer by depositing on said surface a charge of positive polarity and imagewise exposing said photoconductive layer from the side opposite to said charge;

(b) applying negatively charged toner to said electrostatic image to form a toner image;

(c) utilizing said toner image, leaving residual toner on said surface; and

(d) repeating steps (a), (b), and (c) using the same surface the improvement, eliminating a necessity of cleaning said surface between cycles, comprising, after said step of forming an electrostatic image, the added step of redistributing said residual toner from non-image areas to image areas of said surface by brushing said surface with bristles having a triboelectric attraction for said toner in the presence of an electrical field directed away from said surface.

References Cited UNITED STATES PATENTS Steinhelper 101-426 Clark 134-1 Bertelsen 96-1 Steinhelper 96-1 Gundloch 96-1 McNaney 178-66 10 3,167,455 1/1965 Laben et a1 118-637 3,318,284 5/1967 Hojo et a1 118-637 3,405,682 10/1968 King et a1 118-637 FOREIGN PATENTS 1,218,287 6/ 1966 Germany.

GEORGE F. LESMES, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R. 

